The overall goal is to investigate the effects of hypothermia on cardiac regulation. We propose to study cardiac tissue from the bat in active and hibernating stages and to evaluate quantitatively and systematically the dynamics of the transition which occurs during the period of arousal. This evaluation will include myocardial ultrastructure, norepinephrine levels, light and electron microscopic radioautography (L-noradrenaline 7-H3) and cytochemistry. Morphological information will be quantitated at each stage by means of the technique of stereology and will be correlated with biochemical determinations of norepinephrine concentration and radioisotope measurements of norepinephrine turnover. The effects of reserpine, parachlorophenylalanine, and hydroxydopamine injections on these parameters will also be investigated. In addition, cytochemistry will be done at the level of electron microscopy in order to determine the sites of localization of cholinesterase, adenosine triphosphatase, and acid phosphatase as well as seasonal and functional variation in their reactivity.